1. Field of the Invention
This invention relates refractory metal alloys and to powder and other metallurgy processes for consolidating rhenium alloys at reduced temperatures and pressures so that rhenium alloys may be used as coatings for alloys that have lower melting points than rhenium itself, such as steel and nickel alloys, as well as structural shapes where the entire object is made of rhenium alloy.
2. Description of the Related Art
In certain aerospace industry applications, a device""s operating efficiency can be increased by increasing the operating temperature for the device. At these elevated temperatures, strength can be reduced and wear can accelerate for many materials. Wear resistant coatings exist today, but the expense of applying them to conventional substrate materials, such as steel, nickel, and other conventional high temperature alloys, reduces their cost effectiveness.
One group of materials that can have excellent wear rates is refractory metals. However, they are expensive and heavy and so are generally relegated to use as coatings, as whole-part fabrication can be difficult and expensive. Some of such refractory metals have high temperature strength and/or adequate wear resistance but, because their melting temperatures are so much higher than the substrate materials, they can be difficult to use.
Powder metallurgy can be used to fabricate and/or coat cheaper substrate materials with more wear-resistant coatings. Powder metallurgy can use a forming and sintering process for making various parts out of metal powder. After a component has been generally shaped by forming or molding sintering is a high temperature process that can be used to develop the final material properties of the component. It can involve heating the powder to temperatures below the melting point of the major constituent in an inert atmosphere to protect against oxidation. Temperatures of approximately 80% of the melting point of the main constituent material can be used so as not to melt the component and affect its shape. Popular raw materials used in the production of powder metallurgy components are metal powders. These consist of fine, high purity metal powders produced by processes such as atomization, pulverization, chemical reduction, electrolytic techniques or mechanical alloying. Of these processes, atomization is a popular technique. In one process, the metal powder is compacted by injecting it into a closed metal cavity (the die) under pressure. This compacted material is placed in an oven and sintered in a controlled atmosphere at high temperatures, and the metal powders coalesce and form a solid. A second pressing operation, repressing, can be done prior to sintering to improve the compaction and the material properties.
Rhenium (chemical element symbol Re) is one such refractory metal that is useable for powder metallurgy. It melts at 5,741xc2x0 F. (3,172xc2x0 C., 3,445xc2x0 K) and consolidating it by powder metallurgy can occur at approximately 3,272xc2x0 F. (1,800xc2x0 C., 2,073xc2x0 K) and 20,000 to 30,000 psi pressure. Since steel alloys melt near 2,700xc2x0 F. (1,482xc2x0 C., 1,755xc2x0 K) and nickel alloys melt near 2,500xc2x0 F. (1,371xc2x0 C., 1,644xc2x0 K), conventional powder metallurgy techniques generally are not suitable for coating these metal substrates or any others with a melting temperature below the consolidation temperature or with very low strength at these temperatures.
In view of the foregoing, there is a need for a cost effective, robust reduced temperature and/or pressure powder metallurgy process for refractory metals that addresses one or more of the drawbacks identified above. The present invention satisfies one or more of these needs.
This invention dramatically reduces the temperature and pressure required for consolidation of rhenium by including lower temperature constituents that have full or partial solubilities with rhenium. These additions contribute to consolidating the individual rhenium particles to each other most likely through enhanced diffusion and deformability at the particle interface. As a result, the cost is reduced, making the material""s use more cost effective. In addition, these constituents can enhance the oxidation resistance of the alloy. The oxidation resistance and an application for face seal and ceramic encapsulation is described in more detail in incorporated by reference U.S. patent application Ser. Nos. 10/138,090 and 10/138,087. The alloy elements used to date include cobalt, nickel, chromium and manganese. This approach has succeeded and enabled the coating of steel discs for use in face seals as noted in the provisional application referenced above.
An exemplary pressure powder metallurgy process for creating a rhenium alloy may include the steps of providing a first powder of a refractory alloy, providing a second powder of at least a second metal, the second metal being partially or fully soluble with the refractory alloy, mixing the refractory alloy and second powder to provide a mixture, and heating the mixture under pressure to fuse the refractory alloy with the second powder so as to form an alloy in a manner that high temperatures may be avoided, yet the refractory alloy and second powders may be fused.
An exemplary refractory metal alloy produced by the present invention may include is rhenium, where rhenium comprises the largest constituent of the alloy by atomic weight, and a metal selected from the group consisting of cobalt, chromium, manganese and nickel.
Another exemplary metal alloy produced by the present invention may include at least 50% by atomic weight refractory metal, and a metal selected from the group of cobalt, chromium, manganese and nickel.
Other features and advantages of the present invention will become apparent from the following description of the preferred embodiment(s), taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.